Multifrequency voltage generator



May 18, 1954 J. l.. WHEELER MULTIFREQUENCY VOLTAGE GENERATOR 6 Sheets-Sheei. l

Filed April 15, 1953 May 18, 1954 J. L. WHEELER 2,679,014

MULTIFREQUENCY VOLTAGE GENERATOR Filed April l5, 1953 6 Sheeats-SheeI 2 May 18, 1954 J. L. WHEELER MULTIEREQUENCY VOLTAGE GENERATOR e sheets-sheet 5 V Filed April l5, 1953 AAA vvvv

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May 18, 1954 J. L. WHEELER MULTIEREQUENCY VOLTAGE GENERATOR 6 Sheets-Shes?. 4

Filed April l5, 1953 SQMNE.

' I INVENTOR.

Jo/zlz Z. W/ZeC/er May 18, 1954 L. WHEELER 2,679,014

MULTIEREQUENCY VOLTAGE GENERATOR Filed April 15, 1953 e sheets-sheet 5 May 18, 1954 J. L.. WHEELER 2,679,014

MULTIFREQUENCY VOLTAGE GENERATOR Filed April l5, 1955 6 Sheets-Shea?l 6 www N@ Patented May 18, 1954 sli UNITED John L. Wheeler, Rochester, N. Y., assgnor to Stromberg-Carlson Company,I a corporation of New York Application AprilA 15, 1953, Serial No. 348,907

Claims. 1

This invention relates to a voltage generator and, more particularly, to such a generator for producing a plurality of voltages having different frequencies.

Because of the large demand for service on existing telephone facilities, a large number of multi-party subscriber lines are now being utilized. These party lines are common to a plurality of subscriber stations which are selectively rung by ringing voltages of different frequencies. In the past, the voltage generators utilized to produce the plurality of different ringing frequencies were either mechanical or electromechanical in nature, with the consequent disadvantages of lacking stability in operation and requiring a large amount of space for installation. Although some electronic ringing machines are now being utilized, these prior generators of ringing frequency voltages normally utilize narrowly tuned output lters for the purpose of providing the exact frequency necessary in order to selectively energize a particular one of a plurality of subscriber stations on a single subscriber line.

Also, the development of electronic telephone switching equipment has necessitated the design of all electronic ringing circuits which are adapted to selectively provide any one of a number of ringimt frequencies. These circuits, which are intended for use in electronic systems, must be very stable in their operating characteristics, should occupy small amounts of physical space, and should be compatible for use with existing velectrornechanical systems.

Accordingly, one object of this invention is to provide a new and improved voltage generator for producing a plurality of different frequencies for use in signaling systems.

Another object is to provide a new and improved electrcnic multi-frequency voltage generator wherein each of the frequencies produced is not harmonically related to a single frequency source from which the plurality of different frequencies are derived.

Another object is to provide a multi-frequency voltage generator in which each of the plurality of different frequency voltages produced is related to each other in the ratio cf integral multiples of ten.

Another object of this invention is to provide a multi-frequency voltage generator in which the plurality of different frequency voltages produced are not related harmonically to the basic frequency source from which they are derived, and

in which this plurality vof different frequency voltages are not related to each other by any given integral factor of multiplication.

A still further object is the provision of an electronic frequency voltage generator having an accurate and stable output which is controlled by a single standard frequency source.

Another object is to provide a multi-frequency voltage generator in which a plurality of different frequency voltages are produced by adding and subtracting a plurality of standard frequency voltages to produce frequencies which are not harmonically related to any of the plurality of standard frequencies.

Another object is to provide a voltage generator for producing a plurality of different frequencies by utilizing a plurality of independent oscillators which are controlled in accordance with an error voltage derived from the comparison of the output of these independent oscillators with a combination of standard frequency voltages.

Another object of this invention is to provide an electronic multi-frequency voltage generator in which the output from a standard frequency source is divided into a plurality of different harmonically related frequencies which f' are subsequently added to or subtracted from each other and then filtered in a frequency selective network to prevent the production at the output of the generator of frequencies other than those produced by the arithmetical operations on the standard frequency voltages.

A still further object is the provision of a frequency doubling means of stable operating characteristics in which the output of both halves of a multivibrator circuit are added to produce a double frequency-output.

In accordance with these and many other objects, one embodiment of the invention comprises a multi-frequency voltage generator energized by a standard frequency source of oscillatory voltage in which a plurality of cathode connected multivibrator circuits energized by the standard source divide the frequency of the standard source by certain integral factors to produce a plurality of different frequency voltages. Another group of multivibrator circuits, havingI a natural frequency of oscillation near that of the summation or diiference of certain of the different frequency voltages derived from the standard source, produce voltages which are combined at the grid of a non-linear detector with certain combinations of the voltages produced by division cf the frequency of the standard source. The mixing of the standard frequency voltages and the output voltage of the independent multivibrator in the non-linear detector produces a control voltage which is integrated over several `standard frequency' source.

cycles and brator to iereof in accordance "Vith the diilerenecs ber een the output frequency of the independent inulti.vi"rator and the addition or err-:nce frequencies of the combined standar frequencies. T ie non linear detectors also provided th a filteriwork. for the of preventing of the circu V4o -cillate and for :removing of the uncle tile r 'equency components of the control voltage.

A second embodiment of the invention coinprises a harmonic frequency volt re generator in which the output voltage of a standard frequency source is divided into a pli?l lity of voltages having different frequencies lower than that of the standard source by frequency divi ion a plurality of cathode coupled inultivibrators. These different frequencies are individually ill-- tered in a band pass circuit utilising a pair of ainplier tubes which so coupled as to prevent the production of signals having frequencies other than that produced by the frequency division of the source of frequency.

A third embodiment of the invention comprises a decimonic frequency voltage generator in which .a source of oscillatory voltage of a standard frequency energizes plurality of coupled multivibrators for the purpose of dividing this standard frequency by to produce a plurality of different standard quencies which are related in beine integral tiples of ten. of these standard dec .nal frequency voltages is iiltered in hand filter including a pair selectively cou le@ :lier tubes for the purpose of providing output voltage free from frequencies other that produced by the multivibrator division of the This decitmonic frequency voltage generator includes a freou Lcy doubler circuit wherein the output of both p of a multivibrator are combined for the purpose -of providing an output voltage having a frequency twice that of multivibrator producing it.

Many other objects and advantages of this invention will become apparent from consideration of the following specication when t conjunction with the drawings in which:

Figs. l and. 2 are a circuit diagram of synchromonic voltage generator of the present invention;

Figs. 3 and 4i a circuit diagram of har- Inonic voltage generator embodying the invention; and

Figs. 5 6 are a circuit diagram of a deci-- monic voltage generator.

Referring now to l and 2 of the drawings, there is shown a synchromonic voltage ator comprising a standard frequency source the frequency of the output voltage of which in divided into a plurality of different ar.. equencies by a plurality of multivibrator frequency dividing circuits itil, illu, 220. The output frequencies of selected ones of the frequency dividing rnultivibrators ist, 43 and 259i? and 2i@ are combined in one of a plurality of phase detecting circuits lill, illl, itil and Zilli, each of which is also selectively energized by one of a plurality of independent sources of oscillation or multivibrators till, 22 and 26|). The natural frequency of oscillation ci' each of these independent sources is approximately the saine as the summation or difference of the frequencies of the selected of the standard voltages produced by the muitivibrators Cil |00, and M3, 2&0 and 226 which are applied to each of the phase detecting circuits. lt is not necessary for the natural frequencies of oscillation of the multi'vibrators ZIB, 230, 250 or E) to be substantially equal to the sum or difference frequencies since they may be either an integral multiple or subrnultiple of the combined frequencies.

In response to this simultaneous energization by one of the independent multivibrators 2H), 2.36, 25u and 260 and by a selected group of the frequency dividing multivibrators itil, 20,

.140, 200 or 225i, each of the phase detectors provides a control voltage proportional to the difference in phase between the combined standard frequency voltages and the output of the independent sources of oscillation. This control or error voltage is subsequently returned to the associated independent multivibrator for the purpose of varying the frequency thereof to reduce the phase dierential between these voltages. Therefore, the output voltage which is derived from each of the multivibrators 2H), 230, 250 and 269 is controlled in accordance with the combined standard frequency voltages and is accurate and stable.

The standar frequency source 5u which energizes the plurality of frequency dividing multivibrators i60, |26, ille, 20o, and may be of any of the well known types in the art, and, more specifically, may be a 12Go C. P. harmonic ringing oscillator controlled by a tuning fork as disclosed in the copending application of Robert B. Trousdale, Serial No. 348,861, filed April l5, 1953 entitled, Multi-Frequency Voltage Generation and assigned to the .saine assignee as the present application. Although the output frequency of the harmonic ringing oscillator is set at 1290 C. P. S., it should be understood that any output frequency which contains the proper integrally related frequencies may be uti-- lized.

The output voltage from the standard frequency source 5u is coupled to the first frequency dividing multivibrator Hill through coupling condenser 03 and a resistor ill/l and, more specifically, is applied to a resistor it common to `the cathodes of a pair of tubes lili and HB2 forming the multivibrator H19. The multivibrator loll, including the tubes it! and ieg, is of a conventional design wherein the grids of the 'tubes il and mi are cross coupled by a of condensers it and lill which, together with a pair of grid resistors I and i119, determine the natural frequency of operation of the multivibrator E99.

The values of the condensers lli@ and H31 and of the resistors H58 and ist are so designed that the natural frequency of oscillation of the multivibrator it@ is slightly less than 30G C. P. S. or one quarter of the output frequency of the standard source Since the driving or synchronizing voltage coupled to the multivibrator |80 from the standard frequency source 5t is applied to the common cathode resistor itil, this injected voltage tends to stabilize operation of the multivibrator illll at this even integral submultiple of the frequency of the injected voltage. This is true because the injected synchronizing voltage from the standard source 593 is applied to both of the tubes li and W2 in the same phase. Accordingly, the multivibrator i60 tends to operate at a frequency of 300 C. P. S. and is held at this frequency by the injected voltage from the standard frequency source 50.

The output voltage derived from the multivibrator |06 is coupled to the multivibrator |22 through a pair of coupling condensers |23 and |24, and, more specifically, the output voltage from the multivibrator |02 is applied to a pair of cathode resistors |25 |26 of a pair of tubes |2| and |22 forming the multivibrator |223 so that this multivibrator tends to operate at a frequency which is an odd integral submultiple of the frequency of the injected voltage from the multivibrator liill. rlhe multivibrator |22 tends to operate at the odd integral subrnultiple of the 300 C. P. S. voltage coupled thereto from the multivibrator me since the voltages applied to the tubes |25 and l22 are 180 degrees out of phase.

The magnitudes of a pair of grid coupling condensers |21 and |28 and a pair of grid resistors |29 and |43 are chosen so that the natural frequency of oscillation of the multivibrator |20 is slightly less than 60 C. P. S., which frequency is one-fifth of the output frequency of the multivibrator Ill.

The output of the multivibrator 12|] is coupled to the frequency dividing multivibrator |46 through a pair of coupling condensers |44 and |45, and is applied 18() degrees out of phase to a pair of cathode resistors |4 and Ml of a pair of tubes |4| and H32 so that the frequency of the multivibrator i4@ tends to be an odd integral submultiple of the frequency of the multivibrator |20. The grid coupling condensers and the grid resistors of the multivibrator |42 are so chosen in value that the natural frequency of oscillation thereof is slightly less than 20 C. P. S. or one-third of the frequency of the multivibrator |22.

The 60 C. P. S. output of the multivibrator i2@ is coupled to the frequency dividing multivibrator 200 (Fig. 2) through a conductor 43 (Pigs. 1 and 2) and a condenser |22. This voltage is applied to a cathode resistor 263 which is common to the cathodes of a pair of tubes 22| and 202 forming the multivibrator 20G so that this multivibrator tends to operate at a frequency which is an even integral subrnultiple of the output frequency of the multivibrator |22. The grid coupling condensers and the grid resistors of the multivibrator 22s are of such a magnitude that the multivibrator E2G tends to oscillate at a frequency of 3G C. P. or one-half of the frequency of operation of the multivibrator |2il.

The output from the multivibrator 29 is also applied to a cathode resistor 223 of a tube 22l in the multivibrator 222 through a coupling condenser A cathode resistor 225 of a tube 222 forming the other half of the multivibrator 22.6 is supplied with voltage, 1S@ degrees out of phase with that supplied to the cathode resistor 222, through a coupling condenser 225 and a conductor 22T (Pigs. 1 and 2) which'is connected to the plate of the tube |22 in the multivibrator H22. Accordingly, the multivibrator 222 tends to operatc at a frequency which is an odd integral subrnultiple of the 69 C. P. S. output of the multivibrator l2ti. The coupling condensers and grid resistors of the tubes 22S and 222 are so chosen in value that the natural frequency of oscillation of the multivi rater 222 is slightly less than l2 C. P. S. or one-filth of the output frequency of the multivibrator 29.

. Therefore, the plurality of multivibrators Hill, |20, 42, 222 and 223i provide a series of different frequency voltages which are stable in frequency and integral subrnultiples of the 1200 C. P. S. voltage provided by the standard source lill. For gen- 6. eral convenience, the 60 C. P. S., 20 C. P. S., 30 C. P. S. and 12 C. P. S. voltages provided by these multivibrators are designated standard frequencies inasmuch as they are produced under the direct control of the .standard frequency source 50.

The 3) C. P. S. voltage output of the multivibrator 2M is coupled to the phase detector Hi] (Fig. l) through a conductor 235 (Figs. 1 and 2), a mixing resistor |13 and a grid condenser ||4. The 12 C. P. S. voltage output of the multivibrator 222 is also coupled to the phase detector |10 through a conductor 228, a mixing resistor 229 and the condenser ill. These tWo voltages are mixed at the grid of a grounded cathode amplifier tube i|| which functions, by virtue of its grounded cathode, as a non-linear triode detector. The grid of the non-linear detector tube is also provided with voltage from the independent multivibrator 2 l] (Fig. 2) through a conductor 2 |3, a mixing resistor 2 i4 and the condenser H4.

The multivibrator 2 lli includes a pair of triodes 2|| and 2I2, the grids of which are cross connected by a pair of condensers 225 and 2|6. The magnitudes of the condensers 2|5 and 2|6 and a pair of grid resistors 2li and 2 I8 are so chosen that the natural frequency of oscillation of the multivibrator 2|@ is approximately 42 C. P. S. The frequency of li2 C. P. S. is also the summation frequency produced by the addition of the 30 C. P. S. and 12 C. P. S. voltages coupled to the phase detector lib from the multivibrators 200 and 229.

Therefore, when the plurality of voltages provided by the niultivibrators 222, 2li) and 225 are mixed by the resistors i i3, 2 i and 229- at the grid of the zero-bias triode l l i, a direct current error voltage is produced at the output thereof which is proportional to the phase difference between the output of the multivibrator 2id and the combined frequency outputs of the multivibrators its and 22o. The instantaneous value of this error voltage is directly proportional to the phase difference between the above mentioned volt- On the positive portion of the voltage applied. to the grid of the tube lli, a grid current is drawn so that the condenser |14 becomes charged. This charge leaks ofi the condenser l |4 through a grid resistor' Mc during the negative portions of the applied voltage so that the resistor ima. establishes a negative grid bias to prevent the tube l l l from conducting except during a selected positive portion of the applied voltage.

This output voltage from the tube is coupled to a cathode follower tube l2 forming a part of the phase detector network lil through a pair of serially connected resistors H5 and H6. The cathode follower tube H2 couples the error or control voltage applied to the grid thereof to the grid resistors 2li and 2id of the multivibrator 2 is through a conductor 2li-3 (Pigs. 1 and 2). rfhis control voltage applied to the grid resistors 2li Bis varies the frequency of oscillations of the multivibrator 2id or the phase thereof so as to maintain the output frequency of the oscillater 2id equal to that provided by the addition of the 30 and l2 C. P. S. voltages produced by the multivibrators 2532 and 22o.

Inherent in the intermodulation of the voltages applied to the of the tube i l are a number of low frequency intermodulation products. vlt is desirable to prevent these louI frequency interinodulation products from being applied through the cathode follower tube |2 to the in- QTGSS.

dependent multivibrator 2li), and, therefore, an integration network comprising the resistor and a condenser llii is provided at the output of the tube l i The time constant of these two elements is long enough to remove a six C. P. S. signal, and is short enough that the phase detector l |51 is capable of responding to variations in phase in the output of the multivibrator 2 l 0.

The presence of the integrating network provided by the resistors l and i iii tends to reduce the overall gain of the system so that, in order to obtain an adequate magnitude of control volttage on the conductor Zie, it is often necessary to raise the driving voltage coupled to the grid of the tube lll. This increased driving voltage may cause the feedback network including the multivibrator e and the phase detection circuit lll to exhibit a lack of stability, or, in some instances, to exhibit a tendency to oscillate at relatively low frequencies. This tendency to break into oscillation is prevented by the insertion of a bridged T-iilter network H9. The magnitude of the components or" the filter network H9, including the .,stor ile, so chosen that the real component of the product of the fraction of feedback voltage and the amplification of the circuit in the absence of feedback, i. e. the feedback factor, is considerably less than unity at the point at which the phase shift of the system is 180 de- By this choice of the magnitude of the components forming the filter network H9, the feedback network provided by the phase detector Ile and the multivibrator 2|@ exhibits a large degree of stability in operation.

Therefore, the phase detector network IG provides a control voltage proportional to the difference in phase between the output voltage of the multivibrator Zie and the composite voltage produced by the addition of the output voltages of the multivibrators and itt, and this control voltage is utilized to control the frequency of oscillation of the multivibrator 2H] to reduce the phase difference therebetween to a minimum. The output voltage from the multivibrator 2|U which is coupled to an output terminal 2|Ua is both extremely accurate and positively synchroniced with the standard source of frequency 5e.

The i2 C. P. S. voltage output of the multivibrator is also applied to the control grid of a zero--bia triode itil in the phase detection network itil i ugh the conductor 223, a frequency mixing res .,or lei and a grid condenser |34. The grid of the tube itl is also supplied with the 42 C. P. voltage from the multivibrator 2|!) through. the conductor 2lb, a mixing resistor |35, andthe condenser i314.

multivibrator SiS-,ll (Fig. 2) which includes a pair of tubes Ztl nd 232 is provided with grid coupling condenser-s grid resistors of such a magnitude that natural frequency of oscillation of the multivibrator is approximately 54 C. l?. or the sum of the frequencies applied to the grid of the triode from the multivi brators im Zilli. The output voltage of the multivibrator 23! is coupled to the grid of the non-linear detector 3i through a conductor 234 (Figs. l and 2), a mixing resistor 235 and the condenser' litt.

. l The phase detection network llfl including an integrating network ille and a filter network |3l operates in the same manner as the phase detection network lli) to produce an error or control voltage which is coupled through a cathode follower tube |32 and is connected to a pair of grid resistors 236 and 231 through a conductor 238. This control voltage varies the bias on the grids of the tubes 23| and 232 so as to force the multivibrator 23o to operate at a frequency of 54 C. P. S., i. e. to maintain a minimum phase difference between the output of the multivibrator 23|] and the additive component of the 42 C. P. S. and 12 C. P. S. voltages coupled to the grid of the nonlinear detector Isl. Therefore, the output frequency of the multivibrator 230, which is coupled to an output terminal 23M, is equal to the sum of the frequencies of the voltages produced by the multivibrators 2 lil and 22|).

The phase detection network |3il also includes a cathode follower tube 32 similar to the tube y| I2, an integrating network indicated generally as |36, and a band rejection bridged Tnlter network indicated generally as |37.

The phase detection network |59 includes a zero-bias triode 5i the grid of which is provided with 12 C. P. S. voltage from the multivibrator 225 through the conductor 225, a mixing resistor |53 and a grid condenser E54, and also with 54 C. P. S. voltage from the multivibrator 23D through the conductor 231i, a mixing resistor |55 and the condenser I. These voltages are added at the grid of the triode |5| to produce an additive component of 66 C. S.

The multivibrator 259 associated with the phase detector |50 includes a pair of tubes 25| and 252 which are cross coupled by a pair of coupling condensers and are provided with a pair of grid resistors EEES and 251i of such magnitude r that the natural frequency of oscillation of this multivibrator is approximately the 66 C. P, S. produced by the addition of the output frequencies of the multivibrators 23! and 22|?. This output frequency is coupled to the grid of the tube lei in the phase detection network lil through a conductor 255 (Figs. l and 2) and a mixing resistor |56.

The `voltage from the multivibrator 250 and those provided by the multivibrators 22) and -230 are combined at the grid on the non-linear detector tube i5| in the manner described in conjunction with the phase detection networks ||0 land l30 to produce a control voltage proportional to the phase difference between these voltages. This control voltage is integrated in a network |51 and coupled back to the grid resistors 253 and 251i through a lilter network |5, a cathode follower tube !52, and a conductor |53. This control voltage functions in the same manner described above to control the frequency of oscillation of the multivibrator 25e, and thereby to hold the output frequency of this multivibrator vat 66 C. P. S. The output voltage of the multivibrator 250 is coupled to an output terminal 25M.

The multivibrator 25|! provides a 16 C. P. S. output voltage at a terminal 26o@ in accordance with the selected values of a pair of grid coupling condensers and a pair of grid resistors 293 and 254i. The output of the multivibrator is also applied to the grid of a zero-bias triocle 24| in the phase detector circuit 26e through a mixing resistor 265 and a grid condenser 243.

The grid of the detector tube 24| is also provided with 20 C. P. is. voltage from the multivibrator |45 through a conductor |49 (Figsl and 2), a mixing resistor 244 and the condenser 243. The l2 C. P. S. voltage generated by the multivibrator 228 is applied to the grid of the tube 25| through .the conductor 228, a frequency .mixing resistor 245, and the coupling condenser 243.

The third harmonic of the 12 C. P. S. signal and the fundamental of the C. P. S. voltage are `added to provide a subtractive component of 16 C. P. S., the phase of which is compared with the 16 C. P. S. signal from the multivibrator 20d to produce a control voltage having an instantaneous value proportional to the amount of phase displacement. This control voltage is integrated in a network 2% and coupled to a cathode follower tube 24.2 which is connected to the grid resistors 263 and 264 through a conductor 2t? in order to control the frequency of operation of the multivibrator 2'50.

The phase detection network 2li!) does not include a lter of the bridged-T type disclosed in conjunction with the phase detection networks HI), and |59, although such a filtering network could obviously be added to the circuit if it is necessary to prevent oscillations or hunting in the feedback network including the phase detection network 24B and the multivibrator Edil.

Accordingly, the synchromonic voltage generator, disclosed in Figs. l and 2 of the drawings, comprises a means for producing a plurality of different standard frequency voltages which are subsequently combined to control the operation of a plurality of independent sources of oscillation. These independent sources of oscillation produce output frequencies which are equal to the sum or the difference of the standard frequency voltages or an integral multiple or submultiple thereof, under the control of the standard frequency source 50, and, accordingly, provide stable and accurate frequency sources. If extraneous frequencies are present in the output provided at plurality of output terminals wila, 2|Eia, 230m, 2530i and 260e, these output terminals may be connected to the input of individual band pass lters similar to those described hereinafter in conjunction with the voltage generators shown in Figs. 2 to 6, inclusive, of the drawings. These filters convert the generated square wave into sine waves and insure that the plurality of outputs contain only the desired synchromonically related frequencies.

Referring now to Figs. 3 and 4 of the drawings, there is disclosed a harmonic voltage generator including a plurality of frequency dividing multivibrators 30e, 32B, 34u, #tl and i120 which are energized in cascade from a single standard frequency source 290. The plurality of voltage dividing multivibrators produce output frequencies which are controlled in accordance with the standard frequency source 23e and, therefore, are held in synchronism at the desired harmonic frequencies. The plurality of multivibrators 303, 320, 356, itil and 429 energize a plurality of band pass filters 3HE, 330, 35i), Mil and 436, respectively, so that the output frequencies at a plurality of output terminals M, 3Std, 35M, Mila, and 43611 include only the selected harmonic subinultiple frequencies produced under the control of the standard frequency source 280.

In order to provide a rst subdivision of the 1200 C. P. S. standard frequency provided by the source 29e, the multivibrator Sill) is connected to the source 290 through a coupling condenser 303 and a resistor 304 so that the voltage from the source 290 is applied to a cathode resistor 305 which is common to both of the pair of tubes 30| and 302 comprising the multivibrator 3539. A pair of grid resistors 306 and 3111 together with a .pair of grid coupling condensers 308 and 309 are so chosen in magnitude that the natural frequency of oscillation of the multivibrator 300 l is slightly less than 200 C. P. S. Since the synchronizing voltage injected from the standard frequency source 290 is applied to both of the tubes 30| and 302 in the same phase, the multivibrator 360 tends to operate at a frequency which is an even integral submultiple of the frequency of the source Edil and the multivibrator 3539 is thereby synchronized at an operating frequency of 200 C. P. S.

This 200 C. P. S. Voltage output of the multivibrator 300 is coupled to the multivibrator 320 through a coupling condenser 323 and applied to a cathode resistor 324 which is common to a pair of tubes 32| and 322 forming the multivibrator 320. rPhe grid resistors and grid coupling condensers for the tubes 32| and 322 are so chosen in value that the natural frequency of oscillation of the multivibrator 320 is slightly less than 50 C. P. S. so that the injected voltage, which is applied in like phase to both of the tubes 32| and 322, tends to operate the multivibrator 326 at a frequency which is an even integral submultiple of the injected 200 C. P. S. voltage of the multivibrator 30G, i. e. 50 C. P. S.

To provide a third subdivided frequency harmonically related to the frequency of the standard source 280, the output voltage of the multivibrator 32d is applied to the multivibrator 3M through a coupling condenser 3&3 to a cathode resistor 3ds which is common to both of a pair of tubes Sti and .ft2 comprising the multivibrator 34d. The grid resistors and condensers of the tubes 34| and 342 are selected in magnitude so that the multivibrator 340 tends to operate at a frequency of 25 C. P. S., and, since the injected synchronizing voltage provided by the multivibrator 32@ is applied in like phase to both of the tubes 3d! and 342, the multivibrator 340 operates at a frequency of Z5 C. P. S. or an even submultiple of the operating frequency of the multivibrator 320.

To provide a fourth frequency harmonically related to the frequency of the standard source 290, the multivibrator 40d (Fig. 4) is coupled to the output of the 200 C. P. S. multivibrator 3G! through a coupling condenser tilt and a conductor 4M (Figs. 3 and 4i). The voltage provided by the multivibrator Sto is applied to a resistor 405 common to the cathodes of both of a pair of tubes 40| and 032 forming the multivibrator 400. rIhe grid resistors and coupling grid capacitors for the tubes iti and M32 are so chosen in Value that the natural frequency of oscillation of the multivibrator Mill is slightly less than 331/3 C. P. S., and, since the synchronizing Voltage is applied to both of the tubes 40| and 402 in like phase, the multivibrator itil tends to operate at a frequency which is an even integral submultiple of the synchronizing 200 C. P. S. Voltage provided by the multivibrator Std.

To provide a fifth standard frequency harmonically related to the frequency of the standard source 29), the multivibrator d2@ is coupled to the output of the 331/3 C. P. S. multivibrator lill@ through a coupling condenser litt so that synchronizing voltage is applied to a resistor 324 common to the cathodes of a pair of tubes @2| and 22. The grid resistors and grid coupling condensers of the tubes 42E and 22 are of such magnitude that the natural frequency of oscillation of the multivibrator d2@ is approximately 16% C. P. S., and, since the injected synchronizing voltage provided by the multivibrator 400 is applied in like phase to both of these tubes, the multivibrator M0 tends to operate at a fre- 1 l quency which is an even integral submultiple of the 331/3 C. P. S. output of the multivibrator fi-ll, i. e. 162/3 C. P. S.

The output of the 50 C. P. S. multivibrator 320 is coupled to the band pass filter network 3&6 through a coupling condenser 314 and a resistor 315. This voltage is applied to the grid of an amplifier tube 31 i, the output of which is applied to a second amplifier tube 312 in the band pass filter network 310. The tube 312 forms a portion of a cathode follower circuit in order to provide a means for matching the high irnpedance of the multivibrator 321i and the filter 310 with the low impedance of external apparatus, i. e. lines and ringers (not shown), energized by the 50 C. P. S. voltage connected to the output terminal 31M.

In order to prevent the appearance of undesired frequencies at the output terminal 3Min., the plate of the tube 311 is returned to the grid thereof through a parallel-T filter network 313 which is tuned to reject only the desired output frequency or, in this case, an output voltage having a frequency of 50 C. P. S. The filter network 313 provides a degenerative feedback network so that when voltages of a frequency other than that desired appear at the grid of the cathode follower tube 312, these voltages are returned to the grid of the tube 3| l 180 degrees out of phase with the voltages applied thereto to cancel these off-frequency voltages. Therefore, the insertion of the degenerative feedback network including the filter network 313 insures that only an output voltage of the desired frequency will be coupled to the output terminal 31M. In effect, the square waves generated by the multivibrator 32% are converted into sine waves by the output filter 31E) since all of the odd harmonics are cancelled out of the square waves to leave only a residual 56 C. P. S. fundamental.

in order to compensate for the inherent loss of gain because of the insertion of the degenerative or negative feed-back network including the lter 313, the cathode of the tube 31 1 is connected to a cathode resistor 316 which is common to the cathode circuits of both of the tubes 31 l and 312. This provides a positive feedback network for increasing the amplitude of the voltage wave forms produced by the tube 3H and, accordingly, cornpensates for the loss of gain introduced by the degenerative network.

The C. P. S., 331/3 C. P. S. and 162/3 C. P. S. voltage outputs of the multivibrators 3611, 50B and 42d are coupled to the output terminals 33de, 35M, and 5.36ct through the band pass filter networks 330, 360, and 333, respectively, to provide sine wave voltages having a frequency of only the desired value. To accomplish this result, a parallel-T lter network 333 degeneratively interconnects a pair of tubes 331 and 332 in the band pass filter network 3311 and is tuned to reject only voltages having a frequency of 25 C. P. S., a parallel-T filter network 353 degeneratively interconnects a pair of tubes 35| and 352 in the band pass lter network 350 and is tuned to reject only voltages having a frequency of 331/3 C. P. S., and a parallel T-lter i133` tuned to reject 162/3 C. P. S. degeneratively interconnects a pair of tubes i135 and 1322.

In order to provide a 66% C. P. S. outp-ut signal at the terminal 411m, the band pass iilter 411i is interconnected with both plates of the multivibrator me through a coupling condenser 414 and a coupling resistor M5. Since the grid of lan arnplifier tube 41 I in the filter 41e is energized from the outputs of both of the tubes 411 and 402 in the multivibrator 400, the 331/3 C. P. S. output signal of each of these tubes is combined on the grid of the tube 411 to provide a 662/3 C. P. S. signal which is coupled to the grid of the tube 412. A portion of this voltage is returned to the grid of the amplifier tube 411 through a parallel T-lter network 413 which is tuned to reject only 66% C, P. S. so that the degenerative feedback path provided by the filter 413 prevents the coupling of voltages having a frequency other than 662/3 C. P. S. to the cathode follower tube 412 and, consequently, to the output terminal 410e.

Therefore, the harmonic voltage generator, disclosed in Figs. 3 and i of the drawings, provides a plurality of output voltages which are controlled and synchronized by the standard frequency source 290 so as to be quite stable in frequency. Any undesired frequencies are removed from these output voltages by the use of the degenerative band pass filters 31d, 33B, 35e, 410 and litt through which each or" these output voltages is transmitted.

Referring now to Figs. 5 and 6 of the drawings, there is disclosed a decirnonic voltage generator in which a standard source of frequency 490, similar to the source 50 (Fig. l) energizes a plurality of frequency dividing multivibrators 5ml, 5211, 540, iiili and to produce a plurality of standard frequencies which are related in being integral multiples of a decimal basic frequency such as 10 C. P. S. In the embodiment disclosed, output frequencies of 20, 30, e0, 50 and 60 C. P. S. are provided by the plurality of multivibrators 500, 5251i, 54B, B and 62u. The standard frequency signals produced by this plurality of multivibrators are coupled to a plurality of output terminals 52M, Salla, 54017, Sa and 620e through a plurality of band pass filters 51D, 530, 550, 610 and 1539, respectively. These filters function to remove any frequency components from the output voltages which are not the desired decimally related frequencies.

To produce a rst frequency division of the 1200 C. P. S. voltage provided by the standard source dei), the output voltage therefrom is coupled through a condenser v'.503 and a resistor 504 and is applied to a resistor 505 which is common to the eathodes of a pair of tubes 591 and 502. A pair of grid resistors 50S and 5B1 and a pair of grid coupling condensers 568 and 509 are so chosen in value that the natural frequency of operation of the multivibrator 590 is approximately 3010 C. P. S. The multivibrator 550 is synchronized at this operating frequency which is an even subrnultiple of the standard frequency since the injected voltage from the standard frequency source 4.199 is applied in like phase to both of the tubes 5M and 582.

To provide a second multiple frequency controlled by the standa-rd source 490, the output of the multivibrator 530 is coupled to the multivibrator 520 through a pair of coupling condensers 523 and 524. This voltage is applied to a pair of cathode resistors 525 and 5215 for a pair of tubes 521 and 522 forming the multivibrator 52B. A pair of grid resistors and a pair of grid coupling condensers are so chosen in value that the natural frequency of oscillation of the multivibrator 520 is approximately 60 C. P. S. and, since the synchronizing voltage of the multi-vibrator 50|) is applied degrees out of phase to the tubes 52| and 522, this multivibrator tends to operate at a frequency which is an odd integral submultiple of the 300 C. P. S, synchronizing signal.

To provide a third subdivided frequency controlled by the standard source of oscillation 49D. the output of the multivibrator 52D is coupled to the multivibrator 54S) through a pair of coupling condensers and 543. This voltage is applied to a pair of cathode resistors 544 and 545 of a pair of tubes 54 and 542. The grid resistors and grid coupling condensers for the tubes 54| and 542 are so chosen in value thatthe natural frequency of oscillation of the multivibrator 540 is approximately C. P. S. The multivibrator 540 is synchronized at this frequency of operation by the voltage injected from the multivibrator 520 since the application of synchronizing voltages in opposite phase to the pair of tubes 54! and 542 tends to force the multivibrator 540 to operate at a submultiple frequency which bears an odd integral relationship to the injected voltage.

The 300 C. P. S. output of the multivibrator 509 is also appliedI to a cathode resistor B (Fig. 6) common to the cathodes of a pair of tubes Elli and B02 through a coupling condenser 5513 'and a conductor Gilt (Figs. 5 and 6). The grid resistors and the grid coupling condensers for the tubes Bui and @t2 are of such magnitude that the natural frequency of oscillation of the multivibrator ii is approximately 50 C. P. S. and this multivibrator is stabilized at this operating frequency, which is an even integral submultiple of the injected synchronizing frequency provided by the multivibrator 5&0, since the injected voltage is applied to both of the tubes El?! and $02 in like phase.

To produce a fifth frequency division of the standard frequency provided by the standard source tet, the multivibrator 52e is coupled to the output of multivibrator 52@ through a coupling condenser 52'@ and a conductor 523 (Figs. 5 and 6) to apply a voltage to a cathode resistor 624 which is common to the cathodes of a pair of tubes G2! and 522. The coupling condensers and grid leak resistors of the tubes 52! and 622 are of such a magnitude that the natural frequency of oscillation of the multivibrator 52d is approximately C. P. S., and the operation of the multivibrator t2@ at this frequency is stabilized by the application of the synchronizing voltage from the multivibrator 52e to bot-h of the tubes B2 l and 622 in the same phase.

To insure the application of only 60 C. P. S. voltage to the output terminal 52er from the multivibrator 525, the voltage produced by this multivibrator is coupled to an input amplifier tube 5H of the band pass filter network 5I() through a coupling condenser 5i@ and' a resistor 515. This voltage is amplied in the tube 5H and is applied to the grid of a cathode follower tube 512 which provides a means for matching the high impedance of the multivibrator 5225 and lter network 51e with the relatively low impedance of the external apparatus (not shown) to which the output terminal 52e@ is connected. The illter network li also includes a band rejection parallel T-fi ter .Tilt which degeneratively interconnects the plate and the grid of t-he tube 5l l to prevent amplification in the tube 5i l of any signals having a frequency other than the desired 60 C. P. S. voltage. The filter network 54E! also includes a cathode resistor common to both of the tubes 5H and 5i2 to provide positive feedback to the tube 5H to increase the gain thereof as a means for compensating for the loss in gain produced by the parallel T-lter 5|3.

To provide a C. P. S. output at the output terminal 5400i, the lter network 530 is energized through a coupling condenser 534 and a resistor' 535 from the grids of both of the tubes 54| and 542 in the 20 C. P. S. multivibrator 54d. Since an input amplifier tube 53! in the band pass filter 53) is energized with voltage from both of the grids of the multivibrator 5de, these individual grid outputs are combined to produce a 40 C. P. S. signal which is coupled to the output terminal 546 through an impedancematching cathode follower tube A filter network 533 degeneratively interconnecting the plate and the grid of the tube 53E prevents the amplification of extraneous frequencies.

The 20 C. P. S. output of the multivibrator 5M), the 5) C. P. S. output of the multivibrator tilt (Fig. 2), and the 30 C. P. S. output of the multivibrator 52o are connected to the output terminale iilb, tutti.. and Mila through the band pass lter networks 55d, @iii and which function in the same manner as that described in conjunction with the iilter networks 5i@ and 535 to produce output voltages substantially free from undesired frequencies.

While this invention has been described with particular reference to three embodiments thereof, it will be understood that various modifications may be made therein which are within the true spirit and scope of the appended claims.

What is claimed. as new and is desired to be secured by Letters Patent of the United States is:

l. A multi-frequency generator comprising a source of oscillations of a fixed frequency, means for dividing the xed frequency oscillation into a plurality of different lower frequency signals, a plurality of first amplifying means each energized by one of the lower frequency signals, and frequency responsive degeneratively interconnecting the input and output of each of the amplifying means, said frequency responsive means transmitting only frequencies different than the lower frequency energizing the amplifying means to which it is connected whereby each amplifying means produces an output signal of only the energizing lower frequency.

2. A multi-frequency generator comprising means for producing a signal voltage of a fixed frequency, means energized by said fixed frequency voltage for producing a plurality of different frequency voltages lower in frequency than said xed frequency, electron valve means having input and output means, means for energizing the input means of each tube with a one of said lower frequency voltages, a plurality of frequency responsive means interconnecting the input and output means of the amplifying means, each of said responsive means degeneratively returning signal voltages of a frequency different than the lower frequency voltage energizing the amplifying means, and voltage coupling means connected to the output means for providing a plurality of different frequencies.

3. A multi-frequency generator comprising a source of signal voltage of a fixed frequency, means energized by said voltage for producing a plurality of different frequency voltages, a plurality of vacuum tube ampliers having at least a grid and a plate, each of said grids being energized by one of the different frequency voltages, a plurality of filter networks interconnecting the plates with the grids, each of said filters rejecting voltages of the frequency energizing the grid of the tube associated therewith, and a lplurality of cathode follower tubes having grids thereof connected to the plates of the amplifier tubes for providing low impedance output means for each of the dille-:rent voltages.

Li. The generator defined by claim 3 in which the ampliner tube and the cathode follower tuce are provided with a common cathode resistor to produce positive feedback from the follower tube to the ampliner tube.

5. The generator donned oy claim 3 in which the different voltages are harmonically related to the fixed frequency.

6. The generator "cy claim 3 in which the frequencies of the d ferent frequency voltages are integral multiples of ten and lower than the xed frequency.

7. A synchroinonic frequency voltage generator comprising a source of voltage of a fixed frequency, means for converting the fixed frequency voltage to a plurality of voltages of different frequencies, means for combining certain of the different frequencies to produce additive frequency voltages, and means for producing voltages of said additive frequencies under the con trol of the dirierence in phase between the outm puts of the combining means and the producing means.

8. A. voltage generator comprising a source of voltage of a nrst frequency, a source of voltage of a second frequency, means for combining the iirst and second frequency voltages to provide a voltage of a third frequency, means for producing a voltage of a fourth frequency, and means for controlling the voltage producing means in accordance with the phase difference between the third fourth frequencies.

9. A voltage generator comprising a first frequency voltage source, a second frequency volt age source, means for combining the voltage of the first and second frequencies to produce a voltage of a third frequency, means for produca fourth frequency voltage, detecting means for combining the third and fourth frequency voltages to produce error voltage, and means for controlling the fourth frequency in accordance with the error voltage.

l0. A voltage generator for producing a xed frequency voltage from a plurality of standard frequency voltages comprising a source of Voltage of a variable frequency, means for combin ing the variable frequency voltage with the standard frequency to produce a control voltage having a magnitude proportional to the phase difference between the sum of the stand ard frequencies and the variao e frequency, and means for controlling the source of voltage of variable frequency in accordance with the control voltage to produce voltage having an output frequency equal to the o the frequencies of the standard voltages.

l1. A voltage generator for producing a fixed frequency voltage from a plurality of standard frequency voltages comprising a controllable source of voltage ci d frequency, means for combining the standard frequency voltages with the fixed frequency voltage, means for de tectine a phase difference between a voltage produced by an arithmetical ope-ration on the standard frequency 'voltages and fixed frequency voltage, and means for controlling the source of the fixed frequency voltage in accordance with the detected phase difference.

[l voltage generator ier producing a prdete 1mined frequency voltage from a plurality of standard frequency voltages comprising a controllable source of voltage of said predetermined frequency, means for co nlcining the predeterirequency volte nre with the standard frevoltages, for detecting a phase .rence between the sum or dierence or the standard frequency voltages, and the predetermined frequency voltages, means energized by the detecting means to produce a control Voltig magnitude in accordance with said ence for controlling the frequency of tne voltage produced by the source of predetermined frequency voltage, and phase shifting means interposed between the detecting means the controllable source for preventing oscila circuit including said source and said means.

generator for producing a plurality of from a plurality of standard voltages or ha monically related frequencies comprising means for producing a first votage of a frequency not harmonically related to the standard voltages under the control or a number of said Stande ard frequency voltages, means controlled by iirst voltage and another one of said standard frequency voltae'es for producing a second volti ge not harmonically related to the standard frequency voltages.

lll, A generator for producing voltages not related harmcnicaiiy fro-rc a plurality of harmoniy relate derived from a single standard frequency voltage source, comprising a first Allation generator controlled by the combinaically to said combining voltages, means for the frequency of oscillation of said i st generator accordance with the difference phase between the combined voltages and the 'rst Lge, second generating means controlled Ay rst voltage and another of the harmonire...ated voltages for producing a second voltage not relate-i harmonicaily in frequency to the liarmonically related voltages, and means for varying the frequency of oscillation of said second generator in accordance witi the difference in phase between the nrst voltage and the harmonically related voltage controlling the production of the second voltage.

l5. A ringing machine comprising a source of standard frequency voltages, a controllable source of voltage having a frequency, a zero-bias triode, means for applying said first frequency voltage and a voltage representing the result of an a 'netic operation on a selected group of said standard fr quency voltages to the input `of No references cited. 

